What is a blocking variable in statistics?

Blocking is a technique (procedure) that is used in biological / agricultural field experiments to arrange experimental units in groups or blocks that are similar to one another. It is an attemp to improve comparisons of treatments.

Blocking will reduce the source of variability and increase precision (and may be accuracy)

The theoritical basis of blocking is represented b the following mathematical results. Given X and Y are random variables

VAR(X-Y)=VAR(X)+VAR (Y)-2 COV(X,Y)

Phendukani Hlatshwayo (M.Sc Agronomy, Candidate from South Africa)